The soil is a reservoir that holds water for plants. It is important for recharging groundwater by allowing rainfall to infiltrate and filter through the soil into the water table. This happens because the soil is a porous media where the spaces in the soil (pores) are either filled with air or water. The capacity of soil to hold water determines your watering practices in order to provide continued sufficient water for plant growth depending on the drought resistance of your plants. However, not all soils are equal in this capacity.

Bare soil dark with organic matter. Photo by dmp2022.

Field capacity is an important characteristic of soil because it represents the maximum amount of water that a soil can hold. The main soil properties that affect the field capacity of a soil include soil texture, organic matter and compaction.

Soil texture refers to the proportions of sand (0.05-2.0 mm), silt (0.002-0.05 mm), and clay (smaller than 0.002 mm) in the soil. A higher proportion of the larger sand size particles, the more coarse a soil is. Sandy, coarse textured soils have lower field capacity. This is because the greater amount of large particles leads to larger pores from which the water can quickly drain. On the other hand, fine textured soils comprised of the smaller soil particles have a greater amount of small pores that can hold water better, leading to a greater water holding capacity of these soils. However, not all the water held by soils can be used by plants. Some water is very tightly held in the soil and not able to be taken up by plants.

Figure shows general relationship between soil how plant available water capacity (easily available water plus slowly available water) is impacted by soil texture (source: The Nature and Properties of Soils by Ray R. Weil and Nyle C. Brady).

Plant available water holding capacity represents the water retained in soil between field capacity and the wilting point, in another word, it represents the amount of water retained in soil that can be taken up by plants. Even though soils that have high clay contents (such as clay loam, clay) can hold more water, they hold the water tightly so less water is available for plants than silt loam or a sandy loam. The question is, how do we improve soil texture for improved plant available water holding capacity? Unfortunately, changing soil texture of a field is not a viable option unless you introduce foreign soils to the field. Fortunately, there are other options available to improve plant available water holding capacity.

Organic matter is another portion of the soil that holds water. The higher the organic matter content, the more water the soil can hold. Increasing organic matter can increase the field capacity significantly. Increased soil organic matter content improves soil structure which results in increased infiltration, therefore, increasing plant available water. The strategies to increase soil organic matter includes growing cover crops, adding manure, compost or biochar or mixing in leaf mold, peat moss or coir. Avoid disturbing the soil by refraining from overtilling, either manually or with a rototiller.

Till either manually or with a rototiller only when necessary. Photo by dmp2012

Compaction reduces the plant available water holding capacity of a soil. This is because compaction reduces porosity which in turn decreases a soil’s field capacity. Compaction also crushes large pores into smaller pores which leads to a greater proportion of the water being held more tightly by soils. As a result, compaction results in less water available for plants to take up. Also, when a soil is compacted, it becomes harder for roots to penetrate. This can lead to less volume of soil for roots to access to water that is held below than the compacted layer. Stay off soils when they are wet, avoid overtilling, and make defined walking paths through garden areas or perhaps use stepping stones so as not to compact your soils.

Salt content is another soil characteristic that impacts availability of water to plants. Soils that are high in salt concentration tend to have higher wilting points that results in less water for plants to take up. This is more of a problem in the dry regions of the US where salt accumulation is mainly a result of natural soil formation processes and irrigation. In our soils, the salt level can be elevated in areas that have been overfertilized.

In addition to adopting practices that can increase the plant available water holding capacity, there are other practical options for reducing water loss from soil. Mulching is an effective practice to reduce evaporation loss of water. There are many sources of mulches available in the market such as hay, straw, wood chips, bark or cocoa shells. Gardeners can also use untreated lawn clippings.

Haiying Tao, Ph. D.

With the dog days upon us, getting adequate water to our gardens can be a concern. Water is essential to all life. Plants use it to transport nutrients and to maintain turgor – the cellular pressure that keeps soft tissue from wilting.  Plants absorb water (containing soluble nutrients) through their roots and ultimately release it into the atmosphere as vapor through small pores (stomata) on the undersides of leaves in a process called transpiration. Although invisible, the cumulative volume of water transpired by Earth’s plants is prodigious, producing 10% of the atmosphere’s water vapor. One large oak tree can transpire 40,000 gallons per year. Drought stress occurs as transpiration continues and soil moisture is exhausted.

Wilting muskmelon plant
Photo: Erika Saaku, Iowa State

Transpiration rates increase with:

  1. High temperatures
  2. Low humidity
  3. Wind
  4.  More soil moisture
  5. Larger, thinner leaves

Hydrangea or squash leaves wilt on hot, dry, windy days because the transpiration rate of these large-leafed plants is faster than the plant’s ability to take up available moisture from the soil.

At the other end, if a plant’s root system is compromised or undeveloped, extra care must be taken to ensure survival. The process of digging and transplanting exposes roots to the air, damaging or destroying delicate root hairs. Recovery can be difficult with the additional stress of hot weather. Provide shade and plenty of moisture to allow these essential single-cell structures to regenerate. In some cases, cutting back some of the leaf mass to reduce water requirements is advisable.  Old-fashioned advice for transplanting instructs: “water once a day for a week, once a week for a month and once a month for a year.” A very inexact guide to be sure, but a good reminder that transplants have high water demands at first and need to be weaned gradually over time.

Container-grown plants often have root systems a fraction of the size of an equivalent plant growing in the ground. Regular watering is a must, particularly when  containers are made of porous clay or fiber. Potting mixes are commercially available that contain polymer crystals which can dramatically increase the water-holding capacity of potting soils in containers.

Measuring irrigation output

“Deep and Infrequent”

This mantra of watering advice emphasizes the need to train turf grass and landscape plants to develop deep root systems in search for water. Shallow, frequent watering encourages the growth of roots close to the soil surface, making the plant vulnerable to drought stress.  Shrubs and trees with weak, superficial root systems are also more likely to topple over in a windstorm.

Root growth of turf ceases at soil temperatures of about 70°, so lawns should be encouraged to develop deep root systems during the cool weather of spring and fall. Summer watering of lawns is triage; keeping the patient stable until temperatures drop. Overwatering results in excessive growth and increased risk of fungal disease, while wasting water and fertilizers that can potentially contaminate waterways.

Mulch conserves soil moisture as it suppresses weeds and dresses up the garden.  Organic mulches mimic the natural duff on the forest floor, creating a hospitable environment for microbes, fungi, insects and worms as they perform their function of decomposing organic matter and releasing nutrients.

Managing water in the garden is a skill that gets honed over time, as the gardener develops sharper instincts for plant requirements. Water is also a surprisingly efficient and environmentally sound way of ridding plants of some insect pests such as aphids and spider mites – simply knock them off with a forceful spray from the hose. Regular flooding will discourage ground-dwelling bees and wasps (yellow jackets) from nesting in inconvenient areas.

Water makes the garden more pleasurable for people and animals alike. Bird baths, gurgling fountains, lawn sprinklers for children or ponds with fish and frogs create a richer environment and a cool oasis of refreshment on a hot summer’s day.

Children swinging in sprinkler, 1964
Photo: Museum of History & Industry, Seattle

J. McInnis